Alternative High-Level Waste Treatments at the Idaho National Engineering and Environmental Laboratory (1999)

This chapter discusses the sodium-bearing liquid waste currently stored in hanks and managed within the high-level waste (HLW) program at the Idaho National Engineering and Environmental Laboratory (INEEL). This sodium-bearing waste (SBW) is the product of site operations such as decontamination activities, some of which use dilute sodium hydroxide to wash surfaces and solubilize residues. As a result, significant sodium nitrate salts are present in the SBW solutions. For example, waste tank WM-183 contains approximately 4.8 moles of nitrate per liter, in a nitric acid (2.0 M) solution (Law et al., 1997: Table 2, page 5). The cation content, apart from the acid, is 0.7 M Na, 0.6 M Al, 0.1 M K, 0.06 M Fe, 0.04 M Ca, and a small amount of Zr. The radioactive content is about 0.2 curies (Ci) per liter each of ⁹⁰Sr and ¹³⁷Cs, and the actinide activity is approximately 500 microcuries per liter (µCi/l), composed of approximately 350 µCi/l from ²³⁸pu and 125 µCi/l from ²³⁹pu.

The relatively high sodium content makes these solutions unsuitable for direct calcination in their present form because sodium nitrate melts at low temperatures and will not produce a granular, free-flowing calcination product. The SBW can be calcined upon addition of aluminum nitrate. This calcine product has been added to the inventory of HLW calcine already in the bins. However, because SBW does not come from spent nuclear fuel (SNF) reprocessing, it is not initially a HLW but rather a mixed low-level transuranic (TRU) waste that becomes HLW by this mixing with a HLW stream. This current management option for SBW, and future treatment options, cannot be considered apart from classification and disposal issues, as described in this chapter.

The ongoing conversion of low-level SBW to solid HLW by calcination and transfer to HLW storage bins will soon be terminated when the calciner ceases operation on or before June 1, 2000, at least until the calciner is repermitted. The remaining SBW will be consolidated into a few tanks. At this point, the tanks will contain approximately one million gallons of acidic solution and a small but unknown amount of undissolved solids (UDS). This waste is mobile (in liquid form), and it is stored in single-shell tanks located inside concrete vaults. These vaults are not qualified to be an appropriate secondary containment layer under Resource Conservation and Recovery Act (RCRA) land disposal requirements and may not be qualified to withstand seismic activity. In contrast, all of the HLW already has been solidified as calcine and stored in bins that are permitted and designed for a life of 500 years. Therefore, the SBW appears to present a greater current and near-term continuing risk than the calcine,

and the committee believes that the solidification and disposition of this liquid waste must be given a correspondingly high priority. The existing regulatory and other legal drivers clearly reflect this priority as well.

As one approach to treating SBW, the methods considered in Chapter 3 for processing dissolved HLW (calcine) could be used for SBW with little modification. The solid liquid separation (SLS) step will probably require different filter media because solids in SBW are certainly different in composition and physical properties than those arising from leached calcine. Other process steps (removal of TRU and possibly Cs and/or Sr, if necessary) could be essentially identical to those for dissolved calcine. However, one possible difference between separations processes designed for SBW and those designed for HLW calcine would be the effects of organic constituents that may be in the SBW. These organics are presumed to be largely decomposed and/or volatilized during calcination and therefore largely absent from the HLW calcine. The solidification process would not necessarily be the same as for calcine because the radioactivity is lower in SBW and the composition is easier to deal with because it is much lower in Zr, F, and Ca than most of the calcine. In contrast to calcine, SBW is more like low-level waste (LLW) at other Department of Energy (DOE) sites where considerable development work has been done, so there is a large base of information regarding waste treatment, vitrification, and grouting. In general, SBW should pose less of a treatment and disposal problem than calcine.

Although many combinations of processing options were presented to the committee, most of these options treat the SBW as if it were HLW, insofar as both SBW and HLW calcine are processed to develop high-activity and low-activity fractions and material waste forms from these fractions. These high-and low-activity waste forms would be sent to different repositories, a future geologic site in the first case and a U.S. Nuclear Regulatory Commission (USNRC)-or DOE-regulated shallow land burial site in the second.

However, SBW is not HLW, but rather is a mixed low-level TRU waste. R is not clear to the committee why this waste is currently being calcined in a manner which converts it to HLW, nor why other options more appropriate for low-level TRU waste were not considered for the disposal of SBW. Examples of these other options are given in Chapter 12. Disposal requirements defined by the repository are one of only a few sources of boundary conditions that establish processing requirements. Since the SBW is not initially HLW, it is counterproductive to convert the SBW to HLW or to force it into a disposal plan designed for HLW.

An additional argument against calcining the SBW is that this calcination involves an addition of aluminum nitrate, thereby increasing the waste mass. If subsequent dissolution of the calcine were then done to separate radioactive species from nonradioactive species, it would seem that calcination (with the addition of aluminum nitrate) would complicate the separations objectives. Instead, a solidification method that does not add chemicals to the SBW would be preferred from this perspective.

There are two current DOE repositories that are candidate recipients of products of SBW processing. The first is the Waste Isolation Pilot Plant (WIPP) in New Mexico, designated for TRU waste generated in the DOE defense program, but specifically not for HLW. It

may be possible that treatment of SBW would lead to a TRU product that could qualify for acceptance at WIPP.

If a separation step were done to isolate a LLW fraction that could be classified as non-TRU and nonmixed, this LLW fraction could potentially go to a DOE LLW disposal facility such as that at the Hartford Reservation or at the Nevada Test Site (NTS). These Hartford and NTS facilities must comply with applicable RCRA regulations, as must WIPP, which is exempt from the RCRA Land Disposal Restrictions (LDR) but not from waste characterization requirements. There are established Waste Acceptance Criteria (WAC) for the WIPP, Hartford, and NTS LLW disposal facilities (DOE, 1996a; Fluor Daniel, Hanford, Inc., 1998¹; and DOE, 1997). Chapter 9 treats these disposal site options in greater detail.

Several DOE sites are planning to prepare low-level TRU waste for shipment to WIPP. Non-TRU LLW is currently being shipped to NTS from several DOE sites. An example from the Oak Ridge National Laboratory (ORNL) is the detailed consideration of several options for treating nearly one million gallons of LLW that is rather similar to the SBW except that it has been neutralized. Some treatment options involve partitioning into TRU and non-TRU fractions, and some involve direct solidification (e.g., evaporation to a saltcake or incorporation in grout). Recently, a privatization contract was awarded to Foster-Wheeler to complete the solidification and disposition of this waste by 2008. The company plans to separate the alkaline-insoluble solids (sludge) from the supernate, yielding a sludge fraction that is a TRU waste and a supernate that is non-TRU LLW. Both will be evaporated to a dry form. The ORNL plans call for sending the sludge solids to WIPP and the supernate fraction to NTS as a dry saltcake (DOE , 1998b; Brass, 1998).

A simple treatment option similar to this should be appropriate for the SBW. This approach would decouple SBW disposition from calcine treatment and is not complicated by disposition of calcine. Further discussion on such treatment options is contained in Chapter 12.